1. Field of the Invention
The present invention relates to composite flapping flexures. In particular, the present invention relates to the belt stacking sequences in composite flapping flexures.
2. Description of Related Art
In simple terms, flapping is the oscillatory perpendicular movement of a rotor blade relative to the rotor plane in a helicopter or other rotor craft. As those skilled in the art are aware, flapping in rotor blades is necessary to resolve the problem of dissimilarity of rotor thrust. A typical rotor will flap above and below the rotor plane through a total angle of flexure of about 7xc2x0.
For many years, composite materials have been used in the aircraft industry to form parts that are lighter and stronger than their metallic counterparts. Composite materials generally consist of a fiber material, such as fiberglass, suspended in a matrix material, such as an epoxy resin. Helicopter designers employ such composite materials to form rotor yokes that can counteract the cyclic out-of-plane bending loads generated by rotor flapping.
Currently, these composite flapping flexures are manufactured from composite laminates having layers that alternate between load carrying layers and minor load carrying layers. In the load carrying layers, also referred to as xe2x80x9cbelts,xe2x80x9d the fiberglass fibers of the composite material are generally aligned with the longitudinal axis of the flexure. In the minor load carrying layers, also referred to as xe2x80x9coff-axis layers,xe2x80x9d the fiberglass fibers are oriented at selected angles to the longitudinal axis of the flexure, such as xc2x145xc2x0.
Due to fiber orientation, the off-axis layers are softer in bending than the belts. When the flexure is subjected to an out-of-plane bending load, shear strains are generated between the off-axis layers and the belts, the maximum shear strain being at the mid-plane of the flexure. As a result, mid-plane delamination is the predominant failure mode for composite flapping flexures. Because these composite flapping flexures are exposed to cyclic bending, failure can result either from the flexure exceeding a certain maximum flapping angle, or from fatigue due to the cyclic nature of the bending loads.
Although great strides have been made in the design of composite flapping flexures, there is a need for a composite flapping flexure that is more flexible and that has a longer fatigue life.
There is a need for a composite flapping flexure having an increased maximum angle of flexure and an increased fatigue life.
Therefore, it is an object of the present invention to provide a composite flapping flexure having a belt stacking arrangement that increases the maximum angle of flexure and increases the fatigue life of the flexure.
The above objects are achieved by providing a composite flapping flexure in which the load carrying belts are located on the upper and lower surfaces of the flexure, and the non-load carrying off-axis layers are located at the mid-plane of the flexure.
The composite flapping flexure of the present invention provides significant advantages. With a composite flapping flexure according to the present invention, the maximum angle of flexure can be increased and fatigue capability can be improved.